1. Field of the Invention
The present invention relates generally to a spindle motor including a radial bearing and a thrust bearing, both of which are designed in the form of a pneumatic dynamic pressure bearing. More particularly, the present invention relates to a spindle motor of the foregoing type which generates few vibrations during rotation, stands well against shocks imparted thereto from the outside, and exhibits an excellent durability against repeated rotational starts and stops of the spindle motor.
2. Prior Art
FIG. 7 is a fragmentary sectional view of a conventional spindle motor for a hard disc driver (hereinafter referred to simply as an HDD), wherein the left-hand side of the drawing is a vertical sectional view. Referring to FIG. 7, the spindle motor includes a shaft support sleeve 22 which stands upright from a mounting board 21 along a center axis of the spindle motor. A stator coil 23, which consists of a plurality of coils, is fixedly arranged around the outer circumferential surface of the shaft support sleeve 22, and a rotary shaft 25 is rotatably supported around the inner circumferential surface of the shaft support sleeve via ball bearings 24. A support member 27 for fixedly mounting a hard disc thereon is fixedly secured to the upper end Of the rotary shaft 25. A rotor magnet 28 is fixedly secured to the inner circumferential side of the support member 27 facing the stator coil 28. The rotor magnet 28 consists of a multiplicity of radial magnets and is formed in a ring shape.
The magnitude of each vibration induced by the spindle motor having the ball bearings 24 is determined depending on a gap of each ball bearing 24. Specifically, the magnitude of vibration as measured in the radial direction is substantially equal to the radial gap of each ball bearing 24, while the magnitude of vibration as measured in the thrusting direction is substantially equal to the thrust gap of each ball bearing 24. To reduce each of the foregoing gaps, a measure of applying a certain preload to the ball bearings has been hitherto taken. In this case, run-out (oscillation) of each bal bearing 24, as measured in the radial direction is about 0.5 .mu.m in terms of a non-repetition component of the vibration, which fails to exhibit a satisfactory value. When the preload is applied to the ball bearings 24 in that way, the motor torque increases, but this contradicts with the requirement for allowing each HDD to consume a small amount of electricity. For this reason, as far as the ball bearings 24 are used for the spindle motor, it is practically impossible to reduce the magnitude of each vibration induced by the spindle motor much more than the foregoing value.
In addition, a proposal has been made of a spindle motor of the type having a dynamic pressure bearing employed therefor to satisfactorily meet the requirement of exhibiting excellent rotational properties with a high accuracy. FIG. 8 is a sectional view of a conventional spindle motor having a dynamic pressure bearing. Referring to FIG. 8, the spindle motor includes a support shaft 32 which stands upright on a mounting board 31 at the central part thereof. An annular thrust bearing plate 33 is immovably placed on the mounting board 31, and a cylindrical radial bearing member 34 is firmly fitted onto the support shaft 32 in the concentric relationship relative to the support shaft 32. A stator coil 35 is fixedly secured around the support shaft 32 at the positions located below the annular radial bearing member 34 in an equally spaced relationship. On the other hand, a holding member 36 is designed with cap-shaped configuration and its ceiling portion is freely rotatably fitted onto the support shaft 32 at the upper end part of the latter.
A cylindrical radial sleeve 39 is fixedly secured to the inner circumferential surface of the holding member 36 at the central part of the latter, while an annular thrust plate 37 is fixedly fitted to the lower end part of the holding member 36. The inner circumferential surface of the radial sleeve 39 faces the radial bearing member 34 to form a radial dynamic pressure bearing therebetween having a number of herringbone-shaped grooves, while the lower surface of the thrust plate 37 faces the upper surface of the thrust bearing plate 33 to form a thrust dynamic pressure bearing therebetween having a number of spiral grooves formed thereon. A rotor magnet 38 is fixedly secured around the inner circumferential surface of the holding member 36 in an equally spaced relationship at positions located opposite to the stator coil 35.
When the stator coil 35 in the spindle motor constructed in the above-described manner is sequentially activated with an electric current, the holding member 36 having the rotor magnet 38 starts to rotate, whereby a thrust pneumatic dynamic pressure bearing is formed between the upper surface of the thrust bearing plate 33 and the lower surface of the thrust bearing plate 37, while a radial pneumatic dynamic pressure bearing is formed between the outer circumferential surface of the radial bearing member 34 and the inner circumferential surface of the radial sleeve 39. Since the thrust plate 37 and the radial sleeve 39 are borne by the dynamic pressure bearings without any direct contact not only between the thrust bearing plate 33 and the thrust bearing plate 37, but also between the radial bearing member 34 and the radial sleeve 39, the spindle motor can smoothly cope with high speed rotation as compared with the aforementioned conventional spindle motor of the type having ball bearings.